1. Field of the Invention
The present invention pertains to measurement of the diameter of the anterior chamber of the eye and, more particularly, to an instrument for use in such measurement to facilitate selection of an intraocular lens of correct dimensions for insertion in the anterior chamber.
2. Discussion of the Prior Art
Surgical operations to remove cataracts of the eye require removal of the lens thereby permitting the patient to see only vague forms since light will not be focused on the retina at the back of the eye. Accordingly, following cataract eye surgery, some means must be provided to focus light for the patient to have clear vision. For example, cataract glasses can be placed in front of the eye, a contact lens can be placed on the eye, or an artificial intraocular lens can be positioned inside the eye to restore vision. Cataract glasses are effected in restoring straight ahead vision, but have the disadvantage of permitting only limited peripheral vision. Contact lenses are effective in restoring straight ahead and peripheral vision; however, elderly patients, who represent a great majority of those undergoing cataract surgery, have great difficulty inserting and removing the contact lenses on a daily basis. Intraocular lenses provide the most normal visual correction of the above-mentioned means for focusing light in that they restore straight ahead and peripheral vision, do not change the size of objects viewed significantly, and are permanently implanted.
There are essentially three types of intraocular lenses categorized according to their position inside the eye. Posterior lenses are placed behind the iris, while iris supported lenses are clipped, hooked or sewed onto the iris, and anterior chamber lenses are positioned in the anterior chamber of the eye bounded by the corneal dome and the iris plane. Anterior chamber lenses are supported in the anterior chamber by feet positioned at the juncture of the iris with the cornea.
While one size of iris supported or posterior lenses essentially can be utilized for all human eyes since the supports therefor do not abut rigid structures of the eye, the support feet of anterior chamber lenses are placed directly against the sclera; and, thus, anterior chamber lenses must fit the recipient eye exactly. For this reason, manufacturers of anterior chamber lenses typically supply such lenses in 12.0, 12.5, 13.0, 13.5, and 14.0 millimeter sizes, and all of these sizes must be stocked in the operating room so that the correct size implant lens can be used for the recipient eye once the cataracts are removed. If the implant lens is too short for the recipient eye, it can ride forward causing blistering of the cornea, can rotate causing continual inflammation of the eye and can, on rare occasions, cause hemorrhaging. If the implant is too long for the recipient eye, it can cause large amounts of of astigmatism, tenderness to touch and continual inflammation as well as increasing the chances of producing hemorrhage as the implant lens is inserted into the eye. If the wrong size anterior chamber implant lens is inserted into the eye and this is recognized at the time of surgery, the surgeon must remove the lens and insert another implant lens. This exchange is made after the cataract has been removed and when the vitreous is exposed to the air. In this situation, there is danger of the vitreous being lost from the eye. This is a serious complication in cataract surgery. Thus the need to place the correct size implant in the eye on the first attempt. While anterior chamber implant lenses are widely accepted, they pose the additional challenge to the surgeon of correct sizing since the anterior chamber typically varies in diameter from 12.0 to 14.5 millimeters.
The most widely used method of estimating the diameter of the anterior chamber is to measure the horizontal, clear corneal diameter externally of the eye and add 1 millimeter to such measurement under the assumption that the internal anterior chamber diameter is 1 millimeter larger than the external cornea measurement; however, in clinical investigations, it has been determined that the actual implant lens size required was the same as that predicted by this method in only 65% of the cases. In fact, it has been found that the difference between the external and internal measurements do not average 1 millimeter as this method assumes but, rather, the average difference is on the order of 0.6 millimeters and varies greatly from eye to eye, the smallest difference being 0.3 millimeters and the largest difference being 1.2 millimeters.
Another method of estimating the anterior chamber diameter utilizes a ruler-type instrument that is passed through a surgical incision across the anterior chamber until the distal end abuts the juncture of the cornea and the iris, the ruler having a scale in the pupil area such that the surgeon estimates the position of the center of the cornea, and this distance is doubled to obtain an estimate of the diameter of the anterior chamber. This instrument has the disadvantages that it is an indirect method of measurement since only one side of the anterior chamber is contacted and the scale can be difficult to read if there are any air bubbles or blood in the anterior chamber. An improvement over this instrument utilizes a ruler-type instrument passed through the surgical incision across the anterior chamber until its distal end touches the juncture of the iris and the cornea with the measuring scale of the ruler lying at the surgical incision such that the scale is easier to read. This instrument has the disadvantage that it remains an indirect method of determining anterior chamber diameter because the instrument touches on only one side of the anterior chamber and the surgeon must estimate the location of the juncture of the iris and cornea below the incision since it cannot be seen, the location of this juncture varying somewhat with the location of the incision.
Attempts have been made to utilize ultrasonic techniques for measuring the diameter of the anterior chamber of the eye; however, such attempts have had the disadvantages of involving a complicated hand-held method of simultaneously performing an A-scan and a B-scan and, thus, being quite time consuming and requiring much practice.